Boosting the replacement of traditional NH₃ production (Haber–Bosch process) with photocatalytic technology is of great importance for energy and environment remediation. Herein, to develop a photocatalyst with efficient charge separation and abundant reactive sites for photocatalytic N₂ fixation, a biomass-induced diphase-carbon doping strategy is proposed by adding lotus root starch which can be environmentally produced into the preparation of carbon nitride (CN). The adjustment to the CN framework by planar-fused carbon optimizes the band alignment of the catalyst, improving its response to sunlight. In particular, the in-plane-fused carbon in collaboration with the physically piled carbon initiates unique dual electron transfer pathways from different dimensions. The diphasic carbons can both function as qualified reactive sites according to the experimental explorations and further theoretical calculations, which effectively regulate the electron transfer and energy barrier associated with the N₂ reduction on catalyst. The bio-carbon-doped catalyst exhibits drastically enhanced photocatalytic N₂ fixation performance, and the NH₃ yield on the optimized DC-CN0.1 reaches 167.35 µmol g⁻¹ h⁻¹, which is fivefold of g-C₃N₄ and stands far out from the single-phase doped systems. These explorations expand the metal-free skeleton engineering toolbox and provide new guidance for the solar energy utilizations.